Modern baggage scanning systems in airports are increasingly employing computerized tomography (CT) scanning systems to scan and screen packages and luggage for explosive devices. X-ray--based scanning systems typically identify objects on the basis of relative density and radiopacity, whereas other systems can detect such devices on the basis of various other properties, such as atomic number. For example, it is known that explosives generally have a relatively high nitrogen content. Accordingly, a scanner which can distinguish materials on the basis of the atomic numbers of their constituents can be used to detect the presence of explosives.
Plastic explosives, because of their moldability, present a particular challenge to baggage scanning systems because they can be formed into geometric shapes that are difficult to detect. Most explosives capable of significantly damaging an aircraft weigh at least a pound and are sufficiently large in length, width, and height so as to be readily detectable by an X-ray scanner system regardless of the orientation of the explosive within the baggage. However, a plastic explosive powerful enough to damage an aircraft may be formed into a relatively thin sheet that is extremely small in one dimension and is relatively large in the other two dimensions. The detection of such forms of plastic explosives may be hindered because it may be difficult to see a thin sheet of the explosive material in the image, particularly when the sheet is disposed so that it is parallel to the direction of the X-ray beam as the sheet passes through the system.
Thus, detection of sheet explosives in baggage requires very attentive operators. The requirement for such attentiveness can result in greater operator fatigue, and fatigue as well as any distractions can result in a suspected bag passing through the system undetected.
Accordingly, a great deal of effort has been made to design a better baggage scanner. At least one of these designs, described in U.S. Pat. Nos. 5,182,764 and 5,367,552 to Peschmann et al. (hereinafter, the '764 Patent and '552 Patent, respectively), includes a CT scanner of the third-generation type. Such systems have been widely used in the medical imaging arts and typically include an X-ray source and an X-ray detector system secured, respectively, to diametrically opposite sides of an annular-shaped platform or disk. The disk is rotatably mounted within a gantry support so that in operation the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.
The detector system includes a linear array of detectors disposed as a single row in a circular arc having a center of curvature at the focal spot of the X-ray source (i.e., the point within the X-ray source from which the X-rays emanate). The X-ray source generates a fan-shaped beam, or fan beam, of X-rays that emanates from the focal spot, passes through a planar imaging field, and is received by the detectors.
As is well known, a coordinate system is defined by X-, Y- and Z-axes, wherein the axes intersect and are all normal to one another at the "isocenter" (the center of rotation of the disk as the disk rotates about the rotation axis). The Z-axis is defined by the rotation axis and the X- and Y-axes are defined by, and lie within, the planar imaging field. The fan beam is thus defined as the volume of space defined between a point source (i.e., the focal spot) and the receiving surfaces of the detectors of the detector array exposed to the X-ray beam. Because the dimension of the receiving surfaces of the linear array of detectors is relatively small in the Z-axis direction, the fan beam is relatively thin in that direction.
Each detector generates an output signal representative of the intensity of the X-rays incident on that detector. Since the X-rays are partially attenuated as a function of the densities of objects in their path, the output signal generated by each detector is representative of the densities of all the objects disposed in the imaging field between the X-ray source and that detector.
As the disk rotates, the detector array is periodically sampled, and for each measuring interval each of the detectors in the detector array generates an output signal representative of the density of a portion of the object being scanned during that interval. The collection of all of the output signals generated by all the detectors in a single row of the detector array for any measuring interval is referred to as a "projection", and the angular orientation of the disk (and the corresponding angular orientations of the X-ray source and the detector array) during generation of a projection is referred to as the "projection angle". At each projection angle, the path of the X-rays from the focal spot to each detector, called a "ray", increases in cross-sectional dimension from a point source to the receiving surface area of the detector. The density measurement is considered to be magnified, because the receiving surface area of the detector area is larger than any cross-sectional area of the object through which the ray passes. As the disk rotates around the object being scanned, the scanner generates a plurality of projections at a corresponding plurality of projection angles. A CT image of the object may be generated from all the projection data collected at each of the projection angles using well-known algorithms.
To be of practical utility in any major airport, a baggage scanner should be capable of scanning a large number of bags at a very fast rate, e.g., on the order of three-hundred bags per hour or faster, and to provide this rate the scanner must scan an average sized bag at a rate of about 12 seconds per bag or less. CT scanners of the type described in the '764 and '552 Patents take a relatively long to generate the data for a single-slice CT image, because one revolution of the disk requires between about 0.6 and 2.0 seconds. Further, the thinner the slice of the beam through the bag for each image, the better the resolution of the image. Accordingly, the CT scanner should provide images of sufficient resolution to detect plastic explosives on the order of only a few millimeters thick. If 0.6 to 2.0 seconds are required for generation of data for each CT image, and the average bag can be assumed to be about 70 cm long, at the desired throughput rate of 300 bags per hour a conventional CT baggage scanner can only afford to generate an average of six or seven CT images per bag, since the bag must be moved and stopped at each location of a scan. Clearly, one cannot scan the entire bag within the time allotted for a reasonably fast throughput. Generating only six or seven CT images per baggage item leaves most of the item un-scanned.
One solution to this problem involves the use of a two-dimensional detector array. Such an array is typically made up of multiple rows and columns of individual detectors. Relatively rapid three-dimensional imaging can be accomplished using this radiation beam and detector array geometry.
Two-dimensional area detector arrays are disclosed in, for example, U.S. Pat. No. 5,059,800 to Cueman et al. and in pending U.S. application Ser. No. 08/671,716, filed on Jun. 27, 1996 in the name of Bernard M. Gordon, entitled "Quadrature Transverse CT Detection System", and assigned to the assignee of the present invention.
The detectors of the Cueman et al. array include many mosaic scintillation elements separated by a layer of reflective epoxy containing titanium dioxide to reduce optical crosstalk, or interference. One end of each scintillation element is adapted to receive x-rays, while the other end is adapted to transmit light to a photodiode coupled optically to it.
The detector array of the Gordon application, incorporated herein by reference, includes detectors which are longer in one direction than in another so as to provide enhanced image resolution in both the X-Y and Z planes.
Prior art two-dimensional detector arrays continue to suffer from lack of sufficient isolation of adjacent detector elements, which can result in optical and electrical crosstalk, or interference, between them. The small sizes of the individual detector elements and the spaces between them makes it difficult to isolate them sufficiently from one another to prevent such interference. In addition, insufficient alignment of the components and insufficient planarity of the detector arrays decrease the reliability of the detector systems and increase the costs of manufacturing, assembling, installing and replacing them.
Accordingly, it would be an advancement in the art to provide a two-dimensional detector array which minimizes or eliminates the problem of electrical and optical interference within an array of relatively small and closely-spaced detector elements.